Both the blood-brain barrier and the blood-retina barrier are important in controlling the passage of substances to and from the neural parenchyma, especially in the maintenance of homeostasis.
In the retina, the blood-retina barrier is composed of two different types of anatomically separate cells. The retinal vascular endothelium, which supplies the anterior portion of the retina, is currently considered to have an identical structure to the cerebral endothelium (Towler et al., 480 J. Physiol. 10-11P (1994)), whereas the cells of the retinal pigment epithelium cover the permeable vessels of the choroidal circulation and form the posterior barrier by means of their tight apical junctions. Retinal pigment epithelium (RPE) cells are consequently similar to the tight junction epithelial cells of the choroid plexus.
The cerebral and retinal endothelia are of a different nature from the peripheral endothelium. The cells of these epithelia do not serve only to express tight junctions and form a physical barrier. Other properties of the endothelium contribute to the specialized nature of this barrier, such as the distribution and expression of substances such as the glucose transporter (GLUT-1), the transferrin receptor, and P-glycoprotein (Pgp), the expression product of the drug resistance gene. The cerebral and retinal endothelia also differ from the peripheral endothelium in their permeability to the circulating leukocytes.
For in vivo transfer, the use of primary nerve tissues of fetal origin for cellular transplantation in human therapy gives rise to numerous ethical and practical problems. One alternative to this problem is to use primary cell lines of neural origin (e.g., neurons, glial cells, astrocytes) or non-neural cell lines (e.g., fibroblasts, myoblasts, chromaffin cells of the adrenal medulla, hepatocytes). Although the cell lines of the adrenal medulla, fibroblasts or myoblasts can actually release active substances in vivo, such cells are not normally present in the neuroretina of the central nervous system (CNS). Such cells can modify the normal function of the nervous system and cause a rejection reaction.
Because of the heterogeneity of the endothelial cells of different tissues, influenced by the environment of these cells, it is important to be able to have cells adapted to the retinal environment in order to have tools permitting a good morphological and physiological integration of the cells when the cells are implanted or grafted.